The overall goal of the K01 award is to allow Lizhao Wu, Ph.D. to devote full-time effort for research training under the co-mentorships of Drs. Leone, Rosol, and Robinson, to develop into an independent investigator in an academic institution. The retinoblastoma tumor suppressor (RB) gene was the first tumor suppressor identified. While loss-of-function mutations in RB were initially observed in inherited retinoblastoma, inactivation of RB has since been implicated in a wide variety of human tumors. Mounting evidence has implicated Rb in the control of several fundamental biological processes that have direct relevance to tumorigenesis and tumor progression. These processes include proliferation, apoptosis, and terminal differentiation. It was shown a decade ago that Rb is essential for embryonic viability. However, we recently identified a key role of Rb in the placenta that is required for embryonic development and viability because Rb-deficient embryos can be carried to term when their placentas are corrected through genetic or embryonic manipulations. To further understand the biological relevance of this novel function, we propose to achieve the following three specific aims: 1. To determine whether loss of Rb in the placenta is sufficient to induce embryonic lethality and/or pathological defects in wild-type fetuses. Wild-type embryos will be produced with Rb mutant placentas to determine whether Rb is acting in a cell autonomous fashion within the fetus, or if the Rb-associated defects seen in embryonic development are actually due to the observed placental defects. 2. To genetically dissect the role of Rb in different cellular compartments of the placenta. We will conditionally knock out Rb in either the labyrinth trophoblasts or the spongiotrophoblasts to determine in which cellular compartment the Rb function is important for placentation and fetal development. 3. To examine the genetic regulatory networks employed by Rb in the placenta. E2F1, E2F2, E2F3, and Id2 can mediate Rb function in vivo, and concomitant genetic deletion of Rb and either of E2F1, E2F3, or Id2 can suppress many of the defects seen in Rb-deficient embryos. Therefore, placentas deficient for Rb and either of E2F1, E2F2, E2F3, or Id2 will be analyzed to determine whether deletion of any of the four proteins can ameliorate or remedy the Rb placental phenotype. The funding of the K01 award will provide invaluable additional training to the applicant in areas of embryonic manipulation and pathology, and will foster the applicant's career development into an independent investigator in cancer-related research.